In general, referring to FIG. 7, a door driving system includes a door 110 for controlling an air flow through an air passage defined by a case 100, a link mechanism 120 (transmission mechanism) for operating the door 110, and a driving motor 130 for exerting a rotation force at the link mechanism 120. The door 110 is rotatably mounted in the case 100. The link mechanism 120 and the driving motor 130 are arranged outside the case 100.
The driving motor 130 is constructed of a servomotor, in which a DC motor for generating the rotation force, multiple deceleration gears for increasing the rotation force, a potentiometer for detecting a rotation position and the like are built. The servomotor further has a link unit mounted thereout for improving a stop position accuracy of a driving shaft of the servomotor.
However, because both the servomotor and the link mechanism 120 include numerous components, the door driving system occupies a large space outside the case 100. For example, referring to JP-2001-80347A, the link mechanism 120 and the servomotor of width 60 mm×length 80 mm×height 40 mm are to be mounted outside the case 100. Moreover, it is difficult to reduce the door driving system in size and weight.